1. Field of the Invention
The present invention relates to novel ruthenium catalysts, to a process for producing them and to their use for the catalytic hydrogenation of monosaccharides and disaccharides in the preparation of sugar alcohols, with the exception of sorbitol.
2. Description of the Background
The industrial-scale preparation of sugar alcohols is generally carried out by catalytic hydrogenation of corresponding monosaccharides and disaccharides (cf. H. Schiweck et al. “Sugar Alcohols” in Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. on CD-ROM). Catalysts used for this purpose have hitherto been, first and foremost, nickel catalysts, e.g. supported nickel catalysts or Raney nickel. As an alternative, the use of ruthenium-containing catalysts for this purpose has also been reported. In general, the ruthenium catalysts are supported catalysts in which the ruthenium is present on an oxidic or organic support such as carbon.
Thus, U.S. Pat. Nos. 4,380,680, 4,487,980, 4,413,152 and 4,471,144 describe catalysts which are suitable for the hydrogenation of carbohydrates to form the corresponding sugar alcohols and comprise ruthenium on a support material which is stable under hydrothermal conditions. Hydrothermal support materials proposed are alpha-aluminum oxide (U.S. Pat. No. 4,380,680), titanium(IV) oxide (U.S. Pat. No. 4,487,980), aluminum oxide treated with titanium(IV) halide (U.S. Pat. No. 4,413,152) and theta-aluminum oxide (U.S. Pat. No. 4,471,144).
U.S. Pat. No. 4,503,274 discloses catalysts which are suitable for the hydrogenation of carbohydrates to form the corresponding sugar alcohols and are produced by impregnating a support which is stable under hydrothermal conditions with an aqueous ruthenium halide solution and subsequently hydrogenating the solid at from 100 to 300° C.
U.S. Pat. No. 3,963,788 describes ruthenium catalysts which can be used for the hydrogenation of carbohydrates and in which the ruthenium is supported on a specific zeolite based on an aluminosilicate.
U.S. Pat. No. 3,963,789 proposes crystalline aluminosilicate clays, in particular montmorillonite, as supports for ruthenium catalysts.
FR-A 2526782 describes the use of a ruthenium chloride prepared by reaction of sodium chloride and ruthenium via Na2RuCl6 for producing ruthenium catalysts supported on silicon dioxide for the hydrogenation of monosaccharides and oligosaccharides.
The ruthenium catalysts known from the prior art have only moderate reactivities in the hydrogenation of carbohydrates, which results in low space-time yields of sugar alcohols, based on the catalyst used. In view of the high cost of ruthenium, the economics of these processes therefore leaves something to be desired. In addition, the selectivities of the catalysts are not satisfactory, so that an additional outlay is required for isolating the desired products. In particular, epimerization of the hydroxy groups is frequently observed.